Exploring Sarn's surface, The Doctor & Turlough find the interlocking triangle symbol on a column which Turlough identifies as the Misos Triangle from Trion, his home planet. Peri weakens the Master's control over Kamelion, and Kamelion instructs her to remove the comparator circuit from the Tardis, immobilising it. She flees onto Sarn, seeking the Doctor pursued by Kamelion now once again in the control of the Master. The Doctor & Turlough meet the rebel unbelievers who show them some underground tunnels filled with equipment from Trion but the Doctor recognises that the tunnels are volcanic vents and the equipment indicates an eruption is imminent. The Rebels take them to Malkon, the chosen one, who tells them of how he was found in fire nearby and shows them the brand of the chosen one on his arm: the Misos Triangle. Turlough reveals to him he has the Triangle to and together they go to where Malkon was found, the ruins of a crashed ship from Trion that Turlough believes is his Father's ship. Peri finds Turlough & Malkon and tells them that Kamelion is under the Master's control causing them to start back to the Doctor. Timanov returns to the village with the Kamelion Master, who Timanov believes is Logar's Outsider, and orders that the Doctor and Rebels be burnt in the fire.

So Turlough is from the planet Trion, as are at least some of those on Sarn. But what does the Misos Triangle represent? And why had the Master, whose Tardis we see on Sarn, had Kamelion come here? The story is ticking on quite nicely with the threat of a volcanic eruption rumbling in the background to provide some urgency.

This is director Fiona Cumming's fourth Doctor Who story. When her third, Enlightenment, was postponed due to strike action in late 1982 she and her husband took an impromptu holiday to the island of Lanzarote in the Canary Islands. She reported back on the stunning scenery resulting in it being used as Doctor Who's Third overseas location serving as both itself and the planet of Sarn. Another person affected by the strike was this story's writer Peter Grimwade who lost his last directing gig on the show because of it and, as detailed previously, ended up falling out with the producer as a result of a misunderstanding following the cancellation of the final story for season 20. This is a huge shame for the program as his early 1980s efforts show him to be easily the best director the show had had for a long time. Planet of Fire is his last script for the show and he died of Leukaemia on 15th May 1990.